Radiation filter



Feb. 8, 1949. M. o. BASSETT RADIATION FILTER Filed Oct. 18, 1947 2 SM h b mw .n L eon n A now/ %fi y Patented Feb. 8, 1949 RADIQQTION EILTER Mert n 0. 'Basse t; c ctady, N. Y a sll m' to General Electric Company, a corporation of New York Application October 18, 1941, Serial: No. 780 686- 4' Claims. 1

This invention relates to improvements. in radiation filters used in connection with the measurement and indication. of radiant energy.

When it is desired to. detector measure radiant energy of a particular band of frequencies, radiation sensitive devices, such as phototu-bes, are frequently employed as the detecting means. In order that the phototube will be energized by only the desired band of frequencies and remain unafiected by other radiation which may be present, it is: necessary tofil-ter the radiation before it strikes the. cathode of the phototube. For instance. burglar alarm systems responsive to invisible radiation, such as that. in the infrared region of the spectrum, have. been devised. In such systems visible light which would energize the phototube it struck the cathode is prevented from striking the phototube cathode a filtering means.

Heretofore, this filtering means commonly has been of cemented between glass type wherein a filter material cemented between two fiat plates. of glass. When such a filter is mounted. in an assembly.. 2* GQHS'idGIabIe amount of space is consumed, because the filter has a fiat configuration and must. be held imposition by a bracket or similar means of support. Too, very often in a particular detection system it is necessary to screen out all but a narrow band of frequencies, and this done. byemploying more than. one type of filter material in series with the beam of radiation. Commercially available filters of the cemented between glass type normally providcbu-t a single sheet of filter material between each pair; of plates; consequently when a narrow band pass filter is desired, pairs of plates must be superimposed to obtain the requisite effect. It is evident that the addition of plates in this manner would demand an increased amount of mounting space. Another pertinent consideration is that, since the plates are constructed of glass, the filter assemblies would be subject to easy breakage.

Accordingly it is the principal object of this invention to provide a filter for radiation sensitive tubes which may be mounted without the use of brackets and which will occupy a minimum amount of space.

It is another object of this invention to provide a radiation filter which may employ more than one type of filter material withoutincreasing space requirements.

It is a further object of this invention to provide a filter which is unbreakable, inexpensive, and easy to replace.

Th features of the invention which I desire. to protect herein are pointed out with particularity in the appended claims. The invention itsl-zlf together with further objects and advan-v tages thereof may best be understood by refer.- ence to the following specification taken in conneetion with the accompanying drawing, wherein Fig. l.- is a side View of a phototube upon which placed a filter constructed in accordance with the invention and Fig. 2 is a sectional view taken from the section 22 of Fig. l.

In 1 a filter l is positioned upon the glass envelope in of a standard type phototube 2 in such a manner as to surround the sensitive portipn. of a cathode 3. Attached to the cathode 3 and anode in conventional fashion are leads l2 which extend through base element H and provide. means for connecting the phototube into a detection circuit. To prevent stray radiation from being reflected on to the cathode, the purtions of the glass envelope of the phototube 2 which are not surrounded by the filter may be coated with an opaque material such as graphite.

The filter assembly I is constructed from two tubular elements 4 and .5 made of a suitable substantially transparent plastic material of appreciable elasticity or resilience. Examples of such materials are polymerized methyl methacrylate, ethyl cellulose, polystyrene, etc. In av preferred construction I have used celluose acetate from which the tubuar elements may be extruded. Each of the tubular elements has a lengthwise section removed from its wall to permit expansion and contraction of the assembly. Between the two tubular elements is inserted a thin sheet of filter material .5 which may be. selected according; to, the frequencies of radiation one wishes to exclude. The filter material may consist of a sheet of gelatin in which an organic dye suitable ior excluding undesired radiation has been incorporated. A method of preparing this filter material well known in the art comprises mixing a desired oil soluble dye, such as one of the azobenzine derivatives, in liquid gelatin and coating the mixture upon a flat glass plate; whereupon, after the coating has dried, it is stripped from the glass and a thin gelatin filter results. The outer tubular element 5, substantially concentric with the inner tubular element 4 may have such an inside diameter as to provide a tight fit between it and the inner tubular element, thereby assuring that the filter material is held in place. Thus, the outer element 5 may hold the inner element 4 in a stateof compression by virtue of the resiliency of the former. The inner tubular element 4 normally has a quiescent or at rest inside diameter which is slightly less than the outside diameter of the glass envelope of the phototube, and when the filter assembly is expanded and slipped over the envelope of the phototube, the resiliency or elasticity of the assembly insures a tight grip uponthe envelope.

It will be noted from Fig. 2 that a' wider length-' wise section may be removed from the outer tubular element 5 than is removed from the inner,

tubular element 4. This construction permits the overlap of the edges of the inner tubular element "engagesthe envelope of said tube; said radiation and allows cementing of theedges 'l withia suitable plastic cement to obviate relative movement,

of the inner and outer tubular elements. In some instances cementing of the arcuate edges at the top and bottom of the assembly may be advantageous.

The tubular elements s in any convenient manner. It has been found practicable to extrude the tubular elements of proper diameter according to a method of extrusion well known to those skilled in the manufacture of plastic articles. Another practicable method comprises the bending of flat sheets of plastic material to form tubular elements.

A filter constructed in accordance with this invention may be adapted easily to the manufacture of a narrow band pass filter. Since the gelatin filter material a is very thin, two or more sheets may be placed together between the tubular elements without diificulty because no cementing is necessary. Thereupon, the arrangement of two or more filters in series is obtained without the requirement of additional tubular elements. Identification of the particular type of filter construction is achieved by placing a proper label 8 between the two tubularelements.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A radiation filter adapted to engage resiliently a device such as a radiation sensitive tube comprising a radiation transmissible resilient inner tubuiar element having a lengthwise section removed from the wall thereof, a radiation transmissible resilient outer tubular element having a lengthwise section removed from the Wall thereof and adapted to fit tightly over said inner tubular element, and a thin sheet of filter material inserted between the outer surface of said inner tubular element and the inner surface of said outer tubular element.

2. A radiation filter assembly adapted tobe placed over the envelope of a device such as a radiation sensitive tube comprising a radiation transmissible resilient inner tubular element having a lengthwise section removed from the Wall and 5 may be formed H filter assembly comprising a radiation transmissible. resilient inner tubular element having a lengthwise section removed from the wall thereof and having a quiescent inside diameter slightly less than the outside diameter of the envelope of said radiation sensitive tube, a radiation transmissible outer tubular element fitting tightly over said inner tubular element and having a lengthwise section removed from the wall thereof, and attain sheet of filter material inserted between the outer surface of said inner tubular element and the innersurface of said outer tubular element. r

i. In combination: a radiation sensitive tube and a radiation filter assembly which resiliently engages the envelope of said radiation sensitive tube; said radiation filter assembly comprising a radiation transmissible resilient inner tubular element having a lengthwise section removed from the wall thereof to permit expansion and contraction and having a quiescent inside diam eter slightly less than the outside diameter of the envelope'of said radiation sensitive tube; a radiation transmissible resilient outer tubular element having removed from the wall thereof a lengthwise section of greater width than the section removed from said inner tubular element and fittingtightly over said innertubular element, and filter material inserted between the outer surface of said inner tubular element and the inner surface of said outer tubular element.

' MEnTo o. BASSETT.

REFERENCES CITED The following references are of record finthe file of this patent:

UNITED STATES PATENT Number Name Date 2,013,095 Friend Sept. 3, 1935 FOREIGN PATENTS Number 7 Country Date 773,398 France Sept. 3, 1934 

